1. Field of the Invention
The present invention relates to an information recording system for recording information on an optical information recording medium and, more particularly, to an information recording system which can prevent overwrite in an area of an information recording medium on which information has been already recorded.
2. Description of the Related Art
In a conventional information recording system such as an optical disk system for optically recording or retrieving information in or from an optical information recording medium such as a write once or erasable optical disk, a retrieving light beam having a relatively low predetermined level is continuously generated from a semiconductor laser serving as a light source when the system is in a retrieval mode, and the information on the optical disk is retrieved by the retrieving light beam. When the system is a recording mode, a pulsating recording light beam having a relatively high predetermined reference level is generated. The level is intermittently changed in accordance with information to be recorded. Information is then recorded on the optical disk by the recording light beam. In such an information recording system, when a light beam having a level higher than the predetermined reference level is radiated on a recording area of the optical disk, indeterminate information may often be written in the optical disk and destroy the information which was previously written, even if there was no intention to record in that area.
A technique for preventing such destruction of information already written in an optical disk is disclosed in, e.g., Japanese Patent Disclosure (Kokai) No 61-258335. In this system, as will be explained below and as is understood from the waveforms shown in FIGS. 1A, 1B, and 1C, when a pit on the recording medium, i.e., information, is detected by the system when it is in the recording mode, it is determined that overwriting may occur, and the recording operation is interrupted.
FIG. 1A shows a waveform of a light beam reflected from an optical disk which is detected when the system is in a retrieval mode. As is apparent from FIG. 1A, a light beam having a relatively low level is reflected from the optical disk, and low-level components are generated in the retrieving light beam as dark components corresponding to pits on the optical disk. These components are converted into significant data by a processing circuit including a detector.
FIG. 1B shows a waveform of a light beam reflected from the optical disk obtained when information, i.e., data, is being normally recorded in the optical disk, i.e., when data is recorded in a non-recorded area. As is apparent from FIG. 1B, a relatively high-level pulsative waveform corresponding to the recorded data is detected.
FIG. 1C shows a waveform of a light beam reflected from the optical disk when data is overwritten in the optical disk, i.e., when data is being recorded in an area in which data has been already recorded. As shown in FIG. 1C, a light beam with a waveform obtained by superposing the waveform of a reflected light beam in the normal recording mode shown in FIG. 1B onto the waveform of the reflected light beam in the retrieval mode shown in FIG. 1A is generated upon overwriting. In a conventional system, when the waveform of the reflected light beam shown in FIG. 1C is compared with a predetermined threshold level Th which is sufficiently lower than the intensity of the retrieving light beam, and a pit, i.e., a dark component, is detected, it is determined that overwriting has occurred. In other words, it is determined whether an area in which data is being recorded is a non-recorded area or a recorded area in accordance with whether a signal in the recording mode includes a signal corresponding to a pit, thus preventing overwrite.
A signal in the recording mode, in particular, a low-level signal component, is, however, adversely affected by, e.g., undershooting of a signal representing a recording beam intensity. The undershooting may cause the signal as shown in FIG. 1D, to fall below the threshold level Th erroneously, resulting in improper stopping of the recording operation. In addition, a detection error or omission may occur, and overwrite cannot be reliably prevented.